1. Field
An aspect of the embodiments of the present invention relates to an organic light emitting display device and a driving method thereof.
2. Description of Related Art
Recently, various flat panel displays with reduced weight and volume in comparison to a cathode ray tube have been developed. The flat panel displays include a liquid crystal display device, a field emission display device, a plasma display panel, an organic light emitting display device, etc.
The organic light emitting display device displays an image using organic light emitting diodes that emit light by a re-combination of electrons and holes. Such an organic light emitting display device has a rapid response speed and low power consumption.
FIG. 1 is a circuit diagram showing a pixel of a conventional organic light emitting display device.
Referring to FIG. 1, a pixel 4 of the conventional organic light emitting display includes an organic light emitting diode (OLED), and a pixel circuit 2 that is coupled to a data line Dm and a scan line Sn to control the OLED.
The anode electrode of the OLED is coupled to the pixel circuit 2, and the cathode electrode of the OLED is coupled to a second power supply ELVSS. The OLED generates light having a brightness (e.g., a predetermined brightness) corresponding to the amount of current supplied from the pixel circuit 2.
The pixel circuit 2 controls the amount of current supplied to the OLED corresponding to a data signal supplied from a data line Dm when a scan signal is supplied to a scan line Sn. To this end, the pixel circuit 2 includes a second transistor M2 coupled to a first power supply ELVDD and the OLED, a first transistor M1 coupled to the data line Dm and the scan line Sn, and a storage capacitor Cst coupled between the gate electrode and the first electrode of the second transistor M2.
The gate electrode of the first transistor M1 is coupled to the scan line Sn, the first electrode of the first transistor M1 is coupled to the data line Dm. The second electrode of the first transistor M1 is coupled to one terminal of the storage capacitor Cst. Here, the first electrode may be one of a source electrode and a drain electrode, and the second electrode is an electrode other than the first electrode. For example, if the first electrode is a source electrode, the second electrode is a drain electrode. The first transistor M1, which is coupled to the scan line Sn and the data line Dm, is turned on when the scan signal is supplied from the scan line Sn to supply the data signal supplied from the data line DM to the storage capacitor Cst. Here, the storage capacitor Cst is charged with a voltage corresponding to the data signal.
The gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst, and the first electrode of the second transistor M2 is coupled to the other terminal of the storage capacitor Cst and the first power supply ELVDD. The second electrode of the second transistor M2 is coupled to the anode electrode of the OLED. The second transistor M2 controls the amount of current flowing to the second power supply ELVSS from the first power supply ELVDD via the OLED in accordance with the voltage value stored in the storage capacitor Cst. Here, the OLED generates light corresponding to the amount of current supplied from the second transistor M2.
However, the conventional organic light emitting display device as described above has a problem that the voltage value of the first power supply ELVDD varies according to the position of the pixel 2 within the display device due to voltage drop, thereby causing a problem that an image having a desired brightness cannot be displayed.
There has been proposed a method to charge the storage capacitor Cst using a separate reference power supply irrespective of the first power supply ELVDD. The reference power supply does not supply current to the organic light emitting display device, thereby not generating voltage drop. However, when the above described pixel 2 is charged with a voltage using the reference power supply, a problem arises in that the display quality of the entire display panel is deteriorated (for example, generation of spots on the panel) when the voltage of the reference power supply is changed due to external effects.